[arnav] remove dead code identified by audit

Each removal verified by checking actual production callers (frontend,
electron, internal HTTP, MCP-server subprocesses) — not just test
references. Symbols whose only callers were tests are removed along
with those tests.

Production removals (~390 LOC):

- backend/main.py
  - websocket_session: drop `agent:edit_message` WS branch. Frontend
    only ever uses HTTP `POST /api/agents/sessions/{id}/edit_message`
    (frontend/src/shared/state/agentsSlice.ts); nothing on the wire
    sends a WS message of this type.

- backend/apps/agents/agent_manager.py
  - AgentManager._build_connected_tools_context (~80 LOC): zero call
    sites in production; the connected-tools system-prompt context is
    built inline in _compose_system_prompt now.
  - AgentManager._approx_tokens / _summarize_message_block: pure
    helpers whose only callers were tests. The compaction path uses
    LLM-driven _maybe_compact instead.

- backend/apps/agents/browser_agent.py
  - clear_browser_history: only used by tests. _browser_history is
    pruned via the size cap inline.
  - MODEL_MAP constant: never read.

- backend/apps/agents/mcp_preflight.py
  - DISCOVERY_SCAFFOLDING (~25-line system-prompt block): defined but
    never appended anywhere. The header comment described an intended
    use that the codebase no longer has.

- backend/apps/agents/providers/registry.py
  - thinking_params_for, _is_9router_available, OPENROUTER_BASE_URL,
    get_context_window: zero callers in production. Thinking-params
    routing is done by the provider classes directly; 9Router presence
    is detected at request time; context-window numbers are stamped
    onto sessions from BUILTIN_MODELS at launch.

- backend/apps/agents/tools/{base,web}.py
  - BaseTool.get_schema (abstract) + WebSearchTool/WebFetchTool
    overrides: production code in backend/apps/web/web.py instantiates
    these tools and only calls .execute(); the JSON-schema lives in
    the HTTP wrapper, not on the tool class.

- backend/apps/outputs/outputs.py
  - _resolve_model + MODEL_MAP: tests-only.
  - load_output: docstring claimed it was a public helper for "other
    modules" but no module imported it.

- backend/apps/service/client.py
  - set_user_id, the _user_id module global, and the dead cache short-
    circuit in _get_user_id: setter was tests-only. _get_user_id now
    reads user_email directly from settings on every call.

- backend/apps/settings/credentials.py
  - get_provider_credentials: zero callers. The sibling get_agent_sdk_env
    is kept (it has the explicit "Legacy helpers" keep-comment).

Test updates:

- test_agent_manager_unit.py: drop _approx_tokens / _summarize_message_block
  cases (5 tests), update module docstring index.
- test_browser_agent_unit.py: drop clear_browser_history cases (2 tests)
  and the unused _Boom helper class in the repr-fallback test.
- test_outputs_unit.py: drop _resolve_model / load_output cases
  (4 tests), update docstring + import list.
- test_v2_invariants.py: drop get_context_window tests + get_schema
  assertions on web tools (kept name + BaseTool inheritance checks).
- test_service.py: rewrite the 4 set_user_id-driven tests to drive
  user_id through settings.user_email instead, so _get_user_id's live
  envelope-stamping path stays covered.

Verification:
- ruff --select F401,F811,F841 backend/  →  clean.
- pytest backend/tests/ → 1167 passed, 1 deselected (pre-existing
  sandbox git test, unrelated). No tests dropped silently — every
  deletion is paired with the corresponding test removal/rewrite.
- Dead-code scan re-run: dead WS events 1→0, Tier-2 high-confidence
  14→11 (residue is SDK-callback `context` params + Pydantic `cls`
  validators — both false positives vulture can't see through),
  vulture total 165→145.

Total diff: -565 / +34 LOC across 15 files.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_browser_agent_unit.py

@@ -11,8 +11,7 @@ in small helpers that can be exercised in isolation:
     conversations.
   - `_format_tool_result` — translate `ws_manager` result dicts into
     Anthropic content blocks.
-  - `clear_browser_history` / `execute_browser_tool` — small async + state
-    helpers.
+  - `execute_browser_tool` — small async + state helper.
 
 The integration-level tests for `run_browser_agent` /
 `run_browser_agents` / `_create_browser_card` live in the sister file
@@ -40,7 +39,6 @@ from backend.apps.agents.browser_agent import (
     _summarize_messages,
     _trim_history_by_turns,
     _validate_message_pairing,
-    clear_browser_history,
     execute_browser_tool,
 )
 
@@ -59,28 +57,6 @@ def _clear_browser_history_module_state():
     ba._browser_history.clear()
 
 
-# ---------------------------------------------------------------------------
-# clear_browser_history
-# ---------------------------------------------------------------------------
-
-
-def test_clear_browser_history_drops_only_target_entry():
-    ba._browser_history["b-1"] = [{"role": "user", "content": "hi"}]
-    ba._browser_history["b-2"] = [{"role": "user", "content": "yo"}]
-
-    clear_browser_history("b-1")
-
-    assert "b-1" not in ba._browser_history
-    assert "b-2" in ba._browser_history
-
-
-def test_clear_browser_history_missing_id_is_noop():
-    """Calling clear on an unknown id must not raise (it's used in
-    cleanup paths where the cache may already be empty)."""
-    clear_browser_history("never-existed")
-    assert ba._browser_history == {}
-
-
 # ---------------------------------------------------------------------------
 # _hash_tool_call
 # ---------------------------------------------------------------------------